In various known practices in nondestructive object evaluation, a standard object is passed into, through, or by a coil which is suitably excited to induce eddy currents in the standard object. The coil provides output signals indicative of the response of the standard object to the induced eddy currents. Associated detector circuitry effects display of the amplitude and phase of the coil output signals, indicative of standard object parameters such as electrical conductivity. Test objects are then passed into or through the coil and display of test object characteristics is effected to enable comparison of same with the standard object.
Fundamental to the described practice is a setup procedure termed "balancing", i.e., adjusting the testing system such that it operates in use in a preselected subrange of its dynamic operating capability. In this practice, one adjusts the system such that none of its circuits will operate in a saturation range and thereby introduce gross nonlinearities in signal handling. In a manual setup procedure using, for example, Varimac eddy current test apparatus manufactured by Magnetic Analysis Corporation, one observes an oscilloscope display while moving an object through the test coil and manually adjusts a balance potentiometer until an in range condition is observed on the scope. Such manual procedure is cumbersome and can be time consuming.
While the balancing procedure contributes to system accuracy, other considerations bear on performance, one of which is the presence of regions of nominal nonlinear gain in the operative dynamic subrange of the system. Such nominal aberrations result from variations in phase angle between in-phase and quadrature channels from the perfect ninty-degree angle. In this area of what may be termed "calibration", the above-referenced and other known systems either do not have the several required capabilities or implement calibration to a degree which has less than the accuracy desired in certain applications.
From applicant's viewpoint, the present state of the art of object examination through eddy current practices lacks method and system correlative with current microprocessor capability in the area of balancing and calibration and further in ascertaining the validity of generated data. As respects the last item, applicant notes that the art heretofore has not had a validity checking capability in accordance with what might be derived from theoretically determinable parameters, such as electrical conductivity of specimens, given their dimensional and other known character.